Early cardiac pacemakers provided unipolar or bipolar sensing and pacing of a single chamber of the heart, typically the right ventricle, utilizing a pace/sense lead bearing a single electrode or a pair of electrodes, respectively, in contact with the heart chamber. More recently, pacing and/or sensing of both the atria and the ventricles using a pair of pace/sense leads and/or electrodes (unipolar or bipolar) has become common. These techniques typically provide pacing and/or sensing in the right ventricle, using a right ventricular electrode or electrode pair, and the right atrium, using a right atrial electrode or electrode pair, and generally use separate atrial and ventricular pacing leads to locate the electrodes in the respective chambers. This approach is relatively convenient in both epicardial and endocardial approaches, unless there is difficulty in passing two endocardial leads transvenously through the same blood vessels. In addition, it is sometimes difficult to position the atrial electrode(s) in good electrical contact with the atrial heart tissue.
Atrial and ventricular pacing leads typically employ active or passive, distal end fixation mechanisms, which may or may not constitute a distal electrode, to maintain contact of the distal electrode with endocardial or myocardial tissue to ensure adequate stimulation or sensing. For example, such fixation mechanisms include active, retractable/extendable helical coils adapted to be extended and screwed into the myocardium at the desired site and passive, soft pliant tines (of the type described in commonly assigned U.S. Pat. No. 3,901,502 to Citron) which engage in interstices in the trabecular structure to urge a distal tip electrode into contact with the endocardium. The atrial pacing lead may be formed with a J-shaped bend that allows the atrial electrode to be positioned in the atrial appendage and fixed there through use of the fixation mechanism.
Such pace/sense electrodes and distal tip fixation mechanisms are also currently used in conjunction with large surface area cardioversion/defibrillation electrodes extending proximally along the length of the lead sheath for either right atrial or ventricular placement. Separate electrical conductors and connectors are employed to connect the atrial cardioversion/defibrillation electrodes with an implantable pulse generator (IPG) connector terminal for applying cardioversion/defibrillation shock energy to the respective heart chamber.
In these cases, the inter-electrode separation along the lead body and the effective sizes of the electrodes are fixed and not variable in use. In a somewhat related field of cardiomyostimulation, however, it is known to employ a muscle stimulation electrode having a variable length so that it may be surgically threaded through a muscle mass of a given size as disclosed in commonly assigned U.S. Pat. No. 4,735,205 to Chachques et al. Such electrodes, however, have no application in endocardial cardiac stimulation or sensing leads.
In order to avoid the difficulties and expense of implanting separate endocardial atrial and ventricular leads of the types described, it has long been desired to provide a single atrial-ventricular (A-V) lead that can be used to position both the atrial and ventricular pace/sense electrode(s) and, if warranted, a cardioversion/defibrillation electrode, in desired locations in the right atrium and ventricle. A number of such "single pass" A-V pacing leads have been designed over the years as described in commonly assigned U.S. Pat. No. 4,479,500 to Smits.
In one early approach, atrial and ventricular sense or pace/sense ring-shaped electrodes are simply arranged along the outer sheath of the lead and separated apart by fixed inter-electrode distances. In these designs, the proximal electrode(s) is expected to be positioned in the atrium when the distal tip electrode is fixed in the right ventricular apex as described, for example, in U.S. Pat. Nos. 3,903,897 to Woollons, 4,365,369 to Goldreyer and 4,962,767 to Brownlee. Such leads are typically intended for use in a system for sensing atrial depolarizations or P-waves and both sensing ventricular depolarizations or R-waves and applying ventricular pacing pulses to the ventricular apex.
Because internal heart anatomy varies among individuals, it is difficult to obtain a suitable location of the atrial electrode(s) in a position where they will either sense atrial depolarizations or stimulate the atria properly. Consequently, a number of single-pass A-V leads have been designed having atrial and ventricular electrodes which are adjustable relative to one another along the length of a composite lead body. Several designs encase both atrial and ventricular conductors in a common outer sheath with either the atrial or the ventricular conductor within its own sheath and slideably mounted within a lumen of the outer sheath, allowing axial adjustment of the relative positions of the electrodes.
For example, an early single pass A-V lead is taught in U.S. Pat. No. 3,865,118 to Bures, wherein a ventricular lead sheath is slideably mounted within a lumen extending the length of the atrial lead sheath and extends out the distal end thereof. Electrodes are attached to the distal portions of the atrial and ventricular lead sheaths, and electrical connectors are attached to the proximal ends of these sheaths. Coaxial, atrial and ventricular, coiled wire conductors extend through the atrial and ventricular sheaths to the electrical connectors at the proximal ends thereof. Adjustment of the ventricular lead and electrode relative to the atrial electrode therefore results in corresponding adjustment of the axial separation of the ventricular connector relative to the atrial connector. This results in a lead connector that is not compatible with IPG connector elements that are in fixed separation from one another, requiring a special adapter or modification of the lead connector end.
Another early single pass atrial ventricular lead is taught by Sabel in U.S. Pat. No. 3,949,757. In this lead, the atrial lead sheath is slideably mounted within a lumen of the ventricular lead sheath. As with the Bures lead, adjustment of the relative positions of the atrial and ventricular electrodes changes the relative positions of the electrical connectors, with the disadvantages discussed above.
More recent single pass A-V leads that overcome some of the problems of the Bures and Sabel leads are disclosed in commonly assigned U.S. Pat. Nos. 4,289,144 to Gilman and 4,393,883 to Smyth et al. In these A-V leads, the ventricular sheath is slideably mounted within an outer atrial lead sheath. A bifurcated connector assembly with two connector sheaths is mounted to the proximal end of the lead body. The atrial electrode is electrically connected by a fixed coiled wire conductor extending the length of the outer atrial lead sheath to one connector sheath. The proximal end of the ventricular lead sheath slideably extends through the lumen of the atrial coiled wire conductor and through a lumen of the other connector sheath. The distal end of the ventricular lead sheath extends through a side opening in the outer atrial lead sheath at a point proximal to an atrial lead sheath extension, which may have a J-shape. After the electrode separation is adjusted to the patient's heart, the protruding ventricular lead sheath and the ventricular conductor within it are trimmed. An electrical connector pin is attached to the remaining proximal end of the conductor, a time consuming procedure. After attachment, further adjustment of the lead is precluded, as the ventricular sheath and conductor are then fixed.
A further problem common to several single pass A-V leads is that of sealing the lead at the exit points of the inner lead sheath from the lumens in the outer atrial lead sheath or the proximal connector sheath. In the Smyth and Gilman leads, where a coiled wire conductor is exposed to the lumen of the outer atrial lead sheath that the ventricular lead sheath extends through, a fluid path from that lumen to the exterior of the lead raises the risk of current leakage.
In the '500 patent, the Smits lead is provided with an outer lead sheath having an adjustment means for altering the length of the sheath, such as a circumferentially pleated sheath segment or slideably overlapping sheath segments. A first conductor mounted within the outer lead sheath has means for allowing variation of its length, such as a large diameter coiled segment having increased axial flexibility. The outer sheath is slideably mounted around the inner sheath with the inner sheath protruding therefrom. The inner lead sheath is fixed relative to the proximal end of the outer lead sheath so that variation in the length of the outer lead sheath alters the relationship of electrodes attached to the distal ends of the inner and outer lead sheaths. A connector assembly is attached to the proximal end of the outer lead sheath. The outer sheath may be fixed relative to the inner sheath by engageable projections and indentations on the inner and outer sheaths or by a suture. The Smits lead offers a number of advantages as stated in the '500 patent, but the advantages are offset by a complex manufacture of the lead body.
These prior art references are primarily directed to attaining a single pass A-V lead wherein at least sensing of P-waves is assured by proper location of the atrial sense electrode(s) in the atrium when the ventricular lead distal end pace/sense electrode is lodged in the right ventricular apex. In the field of implantable cardioversion/ defibrillation systems, a number of endocardial leads have been proposed or developed for providing atrial or ventricular cardioversion/defibrillation shocks along with sensing of atrial electrical signals as shown, for example, in commonly assigned U.S. Pat. Nos. 4,932,407 to Williams. The Williams leads include a coronary sinus (CS) cardioversion/defibrillation lead having an elongated cardioversion/defibrillation electrode that is intended to be placed into the ostium leading into the CS and blood vessels branching therefrom and more proximally located atrial pace/sense electrode(s) intended to be remain in or near the right atrium for sensing P-waves. The inter-electrode separation between the cardioversion/defibrillation electrode and the atrial pace/sense electrodes is fixed.
In a somewhat related area, interest has existed for many years in achieving electrode positioning for electrical stimulation of specific surfaces of the right atrium or atrial vessel openings adjacent to autonomic nerves or adjacent specific regions of current pathways for a variety of reasons. For example, research has shown that it may be desirable to stimulate parasympathetic nerves in the sino-atrial (S-A) region of the right atrium that influence the atrial heart rate. In commonly assigned U.S. Pat. No. 5,403,356 to Hill et al. (incorporated herein by reference), the stimulation of the triangle of Koch and/or an area of prolonged effective refractory period elsewhere in the atrium for prevention of atrial tachyarrhythmias is also disclosed. By electrophysiological mapping techniques, it is possible to locate optimum sites for electrical stimulation of the atrium. However, it is difficult to place proximal electrodes of permanent endocardial atrial or ventricular or CS leads of the types described above, having fixed inter-electrode spacing, in proximity to these sites due to variations in their locations, the sizes of the heart chambers, the extent to which the cardioversion/defibrillation electrode is extended into the CS, etc.
In all of these cases, it remains desirable to provide at least one relatively movable electrode for achieving a variable inter-electrode separation from a fixed pace/sense electrode(s) or attachment site so that the movable electrode may be placed optimally and be stabilized in the optimal position while avoiding the problems attendant with connection the proximal end of the lead to the IPG connector terminals. It would also be desirable to have the ability to select the exposed surface area of the electrode to enhance sensing characteristics or optimize stimulation energy distribution.